1. Field of the Invention
The present invention relates to a heat-insulating support device for cryogenic equipment, and more particularly, to such a heat-insulating support device which is adapted to be used at cryogenic temperatures.
2. Description of the Prior Art
In the past, there have hitherto been known conventional heat-insulating support devices for cryogenic equipment such as that described in the publication entitled "ADVANCES IN CRYOGENIC ENGINEERING", volume 27, in a chapter titled "MANUFACTURE OF A 6-m SUPERCONDUCTING SOLENOID INDIRECTLY COOLED BY SUPERCRITICAL HELIUM", on pages 109 to 117, published by Plenum Press, New York and London. FIGS. 6 and 7 illustrate such a conventional heat-insulating support device of a cryostat for use with superconducting machine, where FIG. 6 is a schematic cross sectional view of the cryostat and FIG. 7 is an enlarged cross sectional view of a heat-insulating support device of the cryostat shown in FIG. 6.
In FIG. 6, there is schematically shown a cryostat for superconducting machine which comprises a coolant tank 3, a superconducting coil 1 disposed in a coolant tank 3, a coolant 2 such as liquid helium stored in the coolant tank 3 for cooling the superconducting coil 1, a vacuum vessel 4 housing therein the coolant tank 3 for vacuum shielding the coolant tank 3, and a plurality of heat-insulating support means S for supporting the coolant tank 3 to the vacuum vessel 4 in a heat-insulating manner.
As clearly shown in FIG. 7, each of the heat-insulating support means S comprises a bracket 5 fixedly secured to the side wall of the coolant tank 3, a mounting rod 6 mounted on the side wall of the vacuum vessel 4, and a heat-insulating support member 7 formed of a heat insulating material and connected at its one end with the bracket 5 and at its other end with the mounting rod 6. The mounting rod 6 is formed at its one end with an enlarged socket 6a in which the adjacent end of the support member 7 is received and connected thereto as by a pin 6b, and at its other end with a screw thread 6c through which the mounting rod 6 is mounted on the vacuum vessel 4 in the following manner. Specifically, the vacuum vessel 4 has a plurality of annular outward projections 4a formed at its side wall, each of the outward projections 4a having a throughhole 4b which is hermetically closed by a closure member 8. Mounted on the outer surface of the closure member 8 is a spring housing 9 in which a spring means 10 in the form of axially disposed belleville springs having a linear spring characteristic is received. The socket end 6a of the mounting rod 6 is disposed in the throughhole 4b in the vacuum vessel 4 with its threaded end 6c extending outward through the closure member 8 and the bottom 9a of the spring housing 9. Slidably fitted on the mounting rod 6 is a presser member 11 which has a sleeve 11a and a flange 11b integrally formed with each other. An adjusting nut 12 is threadedly engaged with the threaded end 6c of the mounting rod 6 and abuts at its one side against the flange 11b of the presser member 11 so that the mounting rod 6 is connected with the closure member 8 through the nut 12, the presser member 11, the belleville springs 10 and the spring housing 9. By turning the adjusting nut 12 in a tightening or loosening direction, the presser member 11 is caused to displace axially so as to compress the belleville springs 10 or permit them to expand whereby the resilient force of the belleville springs 10 can be adjusted in an appropriate manner. A dust cover 13 is attached to the closure member 8 for protecting the heat-insulating support device S from the outside.
Reference numerals 14 and 15 designate an O ring disposed between the closure member 8 and the outer peripheral surface of the mounting rod 6 and another O ring disposed between the outer surface of the projection 4a on the vacuum vessel 4 and the inner surface of the closure member 8, respectively, for hermetically sealing the interior of the vacuum vessel 4 from the outside.
In the above-described manner, the superconducting coil 1 is fixedly mounted on the bottom of the coolant tank 3 which is, in turn, supported by the vacuum vessel 4 through the heat-insulating support means S. With this construction, the coolant tank 3 is at room temperature when no coolant is stored therein, but cooled down to a cryogenic temperature of about 4.2.degree. K. (-269.degree. C.) when supplied with a cryogenic coolant 2 such as liquid helium so that it is subjected to tremendous thermal contraction. Due to such thermal contraction of the coolant tank 3, there will arise a great tensile force acting between the heat-insulating support member 7 and the mounting rod 6, which is to be absorbed by means of the belleville spring 10. On the other hand, the belleville spring 10 is given an appropriate amount of tension such that it can prevent any slight displacement of the coolant tank 3 due to relatively limited external forces acting on the vacuum vessel 4. Also, by turning the adjusting nut 12, the coolant tank 3 can be displaced relative to the vacuum vessel 4 whereby the position of the superconducting coil 1 can be adjusted in an appropriate manner.
With the conventional heat-insulating support device as constructed in the above manner, however, the preset position of the superconducting coil 1 tends to be displaced considerably beyond an allowable range by a great force such as an eccentric electromagnetic force acting on the coolant tank 3. In order to prevent such a situation, it is considered that the spring constant of the spring 10 be made greater so as to reduce the amount of displacement of the coolant tank 3 to be within a prescribed allowable range. In this case, however, it is difficult to absorb the tensile force created by the thermal contraction of the coolant tank 3 and therefore it is necessary to increase the cross sectional area of the heat-insulating support member 7 and the mounting rod 6 for improved mechanical strength. As a result, the amount of heat transferred from the vacuum vessel 4 of room temperature toward the coolant tank 3 through the mounting rod 6 and the heat-insulating support member 7 increases, thus deteriorating the heat-insulating capability thereof.